I agree - you can't use "marger" here. However, "margins" here imply a defined border between the text to be "read" and the rest of the page - perhaps "kant" would suit here? And the purpose of the blinds (an unfortunate term to use in this context, but it would be difficult to find a good alternative) is to hide the text, hence my suggestions. These are purely "home-made" attempts - could not find anything related via Google.